usb: typec: tcpm: fusb302: Use 'gnu_printf' format notation

Fixes the following W=1 kernel build warning(s):

 drivers/usb/typec/tcpm/fusb302.c: In function ‘fusb302_log’:
 drivers/usb/typec/tcpm/fusb302.c:186:2: warning: function ‘fusb302_log’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
 186 | _fusb302_log(chip, fmt, args);
 | ^~~~~~~~~~~~

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Yueyao Zhu <yueyao.zhu@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200703174148.2749969-23-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/typec/tcpm/fusb302.c

@@ -178,6 +178,7 @@ static void _fusb302_log(struct fusb302_chip *chip, const char *fmt,
 	mutex_unlock(&chip->logbuffer_lock);
 }
 
+__printf(2, 3)
 static void fusb302_log(struct fusb302_chip *chip, const char *fmt, ...)
 {
 	va_list args;